It's impossible to be 100% sure, but proof marking dates back to blackpowder days and I'm not aware of any country in Europe that didn't do it. I know that at least some guns in both Sweden and Switzerland were proofmarked dating back well before WWII. That said, it's not possible to be 100% certain that none slipped through and I don't know what the rules are regarding proofmarking prototypes in all the European countries.Are you sure?
It's all a guess, there's nothing 100% sure in any of this. My guess is that it was not made in 1944 because that seems like an implausible time for people to be making high-quality single shot pistols virtually anywhere in the world. My guess is that it was not made in Europe because of the practice of proofmarking guns that seems to be nearly universal in European countries. My guess is that it is a one-off/prototype gun, made in the U.S., by an individual, well after WWII.